The goal of this project is to exploit the accurate Monte Carlo based software we have developed to improve electron-beam and, to a lesser extent photon-beam radiotherapy and to make these improvements available to other centers. In our in-place grant we have developed a fully 3- dimensional electron beam dose calculation system which uses Monte Carlo techniques. This consists of two major components - a fast, accurate dose calculating component which is integrated into an existing 3-D treatment planning system and a flexible code, called BEAM, for simulating radiation transport in complex linac heads. In the next grant cycle, a benchmark of clinical impact will be performed, research on optimization of electron beam radiotherapy will begin, and improved dosimetry will be investigated which will use the more precise beam characterization provided by BEAM. We will make the software developed under the in-place grant more user- friendly and make it available to other interested parties. We will also simulate electron beams from a variety of other accelerators beyond the two funded in the in-place grant and benchmark these simulations so that the dose computation algorithm can be used for electron beam treatment planning using these accelerators. A major goal of the next phase is to extend the technique to photon accelerators and photon beam treatment planning. The simulation of photon accelerators, including those with multileaved collimators, will make high quality photon spectra available for advanced treatment planning algorithms such as the convolution method. Dose computation using Monte Carlo techniques will be a valuable tool for improving/benchmarking advanced algorithms and for occasional use in difficult clinical situations. The five major components of the project in the next cycle are: 1) Extension of the accelerator simulation code. 2) Improvement of electron beam dosimetry. 3) Evaluation of the clinical importance of Monte Carlo planning. 4) Improvement of clinical electron beam radiotherapy using optimization techniques. 5) Extension of accelerator and dose computation software to photon beams.